Semiconductors are characterized by their forbidden band or gap separating the last filled states of the valence band and the following empty states in the conduction band. Among the semiconductors, one can distinguish between semiconductors with a small gap, such as silicon and germanium, and semiconductors with a big gap such as, for example, GaN and SiC.
At present it is extremely difficult or even impossible to obtain solid substrates in a semiconductor with a big gap. In the case of GaN for example, no solid substrate of electronic quality exists despite intense research carried out in this field. On the other hand, hetero-epitaxial GaN on a solid substrate in sapphire or in SiC exists. This technique was developed for production of blue diode type optoelectronic components.
Nonetheless, epitaxy of GaN on sapphire is made particularly delicate because of the difference in lattice structure existing between GaN and sapphire (of the order of 16%). Therefore, obtaining crystalline layers of sufficiently high quality for producing optoelectronic devices requires perfecting sophisticated epitaxy methods. The use of the sapphire substrate is essentially explained by its structural and chemical compatibility with GaN, its low cost and its availability under the form of large diameter substrate. The electrically insulating property of sapphire requires production, in the epitaxial GaN, of horizontal components with electrodes located in the front face.
The other method used for retail components is that of GaN on a solid SiC substrate. SiC substrates remain rare and very expensive. This is the method developed and marketed by the company of Cree Research Inc., profiting from the advantage it has of retailing the major part of SiC substrates. The interest of solid SiC for epitaxy and the production of devices with a GaN base is evident. First of all, the low difference in lattice structure (3.5%) between SiC and GaN makes it possible to simplify epitaxy methods while still producing layers with high crystalline quality. Furthermore, the use of a conducting SiC substrate makes it possible to produce a vertical component for passing current (that is, with an electrode on each face). This structure enables production of components of smaller size than those produced on an insulating substrate, which is of interest from the economic point of view. Moreover, the use of SiC, with its high thermal conductivity, makes it possible to adjust or lower the component temperature during its operation. This is an important point as far as performance, service life and reliability are concerned.
Other methods are also being studied, but their present state of development restricts them to laboratory use. The general approach consists of using a substrate of solid silicon in order to benefit from the low cost and large size of these substrates. Thus one can obtain GaN on SiC covering silicon. These techniques, developed under laboratory conditions, rely on the use of a film of epitaxial cubic SiC either on an SOI substrate or directly on a solid silicon substrate. This SiC layer must make it possible to facilitate epitaxy by reducing the difference in the lattice parameter between the GaN and the silicon, that is to arrive at a configuration of epitaxial GaN on SiC. Apart from the problem of producing epitaxial GaN, the first epitaxy of SiC poses significant technical problems. However, the growth of GaN on such a structure is of particular interest because it would make it possible to obtain GaN with cubic structure (sapphire obtains a hexagonal structure) which, because of its properties, is interesting for optoelectronic applications. For the moment this method is still at the research stage.
Finally, a more recent method relates to the direct epitaxy of GaN on silicon, without any buffer film of SiC. For this, one uses silicon (111). This approach, based on a principle equivalent to that adopted for epitaxy on sapphire, suffers at present from being far behind, relative to other techniques. Nonetheless, correct control of the material silicon makes it possible to envisage using electrically insulating or conducting epitaxial support substrates, leaving a certain freedom for the operating mode of the epitaxied device (vertical or horizontal).
The optoelectronic components produced on the above-mentioned materials therefore have either a lateral structure (with two electrodes located on the front face of the substrate), or a vertical structure with one electrode on the active layer (generally in GaN) and another electrode on the rear face of the solid substrate (in SiC, for example). According to the structure adopted, or imposed by the nature of the substrate, the size of the chip evidently varies. From a strictly economic point of view, the production of a vertically operational chip is clearly more advantageous because it makes it possible to produce more compact devices.
Furthermore, the nature of the substrate chosen for epitaxy has an influence on the performance of the device via the problem of heat dissipation during operation. From this point of view, solid SiC has a considerable advantage. The limitations recognized concerning devices produced on GaN supported by sapphire, are under study at present. Two methods are described in publications concerning the solution of this problem for sapphire. Each depends on eliminating the sapphire substrate after producing active epitaxial layers.
The first method depends on eliminating the sapphire substrate and producing a thick epitaxy of GaN (greater than 100 μm) at the rear face in order to obtain a self-supporting rigid membrane. This again means producing a GaN substrate. This approach makes it possible to produce a device with vertical operation and to dissipate the generated heat.
The second method depends on eliminating the sapphire substrate and adhering the active layer onto an electrically and thermally conducting sole (adhesion on a copper substrate, for example). Thus it would be possible to obtain a vertically operational device enabling dissipation of the produced heat.
Thus it can be understood that the SiC approach represents a very promising future for developing optoelectronic branches with a GaN base. The trend for growth techniques other than those on SiC is to produce devices with vertical current flow and to eliminate the generated heat as much as possible during operation of the device, whatsoever the epitaxial support. In the case of epitaxy on sapphire, this substrate only plays the role of epitaxial support and no longer limits the operation of devices because it can be eliminated.